A conventional ultrasonic flowmeter is described in, for example, Patent Reference 1. FIG. 6 is a control block diagram showing a first example of the conventional ultrasonic flowmeter described in Patent Reference 1.
In the ultrasonic flowmeter of FIG. 6, a first oscillator 5 for sending ultrasonic waves on the way to a fluid pipe line 4 and a second oscillator 6 for receiving the ultrasonic waves are arranged in a flow direction. Also, the ultrasonic flowmeter comprises a sending circuit 7 to the first oscillator 5, and an amplification circuit 8 of a signal received by the second oscillator 6. Then, the ultrasonic flowmeter has a configuration in which a signal amplified by the amplification circuit 8 is compared with a reference signal by a comparison circuit 9 and time from sending to receiving is obtained by a time counting unit 10 such as a timer counter and a flow rate value is obtained by a flow rate calculation unit 11 in consideration of a state of a flow or a size of the pipe line according to its ultrasonic propagation time and timing of signal sending to a trigger unit 13 of the sending circuit 7 is adjusted by a value of this flow rate calculation unit 11.
Next, its operation will be described. A burst signal is sent out of the sending circuit 7 based on instructions from the trigger unit 13 and an ultrasonic signal sent by the first oscillator 5 according to this burst signal propagates through a flow and is received by the second oscillator 6. Then, the received signal is processed by the amplification circuit 8 and the comparison circuit 9 and time from sending to receiving is measured by the time counting unit 10.
When a sound in static fluid is set at c and a speed of a fluid flow is set at v, an ultrasonic propagation speed of a forward direction of the flow becomes (c+v). When a distance between the oscillators 5 and 6 is set at L and an angle between the ultrasonic propagation axis and the central axis of a pipe line is set at φ, ultrasonic arrival time T is as follows,T=L/(c+v COS φ)  (1)and the following formula is obtained from the formula (1),v=(L/T−c)/COS φ)  (2)and when L and φ are known, a flow speed v is obtained by measuring T. From this flow speed, a flow rate Q is obtained by the following formula when a passage area is set at S and a correction factor is set at K.Q=KSv  (3)
FIG. 7 is a control block diagram showing a fourth example of the conventional ultrasonic flowmeter described in Patent Reference 1. In the ultrasonic flowmeter of FIG. 7, repeats of sending to receiving are done by the number of repeats set in a repeat setting unit 16 by a repeat unit 15 and switching between sending and receiving is further performed by a switching unit 17 and thereafter, repeats are similarly done. That is, when ultrasonic waves are generated from a first oscillator 4 by a sending circuit 7 and the ultrasonic waves are received by a second oscillator 5 and a received signal arrives at a comparison circuit 9 through an amplification circuit 8, the sending circuit 7 is again triggered by a trigger unit 13 by instructions of the repeat unit 16. This repeat is done by the number of repeats set in the repeat setting unit 15 and when the number of set repeats is reached, time taken to do the repeats is measured by the time counting unit 10. Thereafter, sending and receiving of the first oscillator 4 and the second oscillator 5 are connected in reverse by the switching unit 17 and in turn, ultrasonic waves are sent from the second oscillator toward the first oscillator 5 and in a manner similar to the above, arrival time is obtained and this difference is obtained and a flow rate value is calculated by the flow rate calculation unit 11.
When a sound in static fluid is set at c and a speed of a fluid flow is set at v, an ultrasonic propagation speed of a forward direction of the flow becomes (c+v) and a propagation speed of a backward direction becomes (c−v). When a distance between the oscillators 7 and 8 is set at L and an angle between the ultrasonic propagation axis and the central axis of a pipe line is set at 4) and the number of repeats is set at n, respective repeat times T1 and T2 of the forward direction and the backward direction are as follows,T1=n×L/(c+v COS φ)  (4)T2=n×L/(c−v COS φ)  (5)and the following formula is obtained from the formulas (4) and (5),v=n×L/2COS φ×(1/T1−1/T2)  (6)and when L and φ are known, a flow speed v is obtained by measuring T1 and T2.
However, when a flow rate is small and the number of repeats is small, a difference between T1 and T2 is extremely minute and it is difficult to accurately measure the difference, so that the number of measurements is largely set and an error is relatively decreased. Also, when the flow rate becomes large, the difference of T1-T2 also becomes large, so that it becomes easy to measure the difference and in that case, the number of repeat settings is decreased and a sampling interval is quickened and the error is decreased. That is, the number of repeats of the repeat setting unit 15 is changed by the flow rate calculation unit 11.
The ultrasonic flowmeter of this Patent Reference 1 has a method for switching between sending and receiving using two oscillators and obtaining a flow speed from ultrasonic propagation times obtained from respective received waveforms and calculating a flow rate.    Patent Reference 1: JP-A-8-122117